Thermostats for heating, ventilation and air conditioning systems (HVAC) sense the temperature of a room or office and provide a signal to the system to cool or heat the occupancy space in accordance with a prescribed temperature set point. When the temperature of the occupancy space rises above or below the set point, an appropriate signal is provided to the HVAC system which in response heats or cools the occupancy space.
To reduce the costs of heating and cooling an occupancy space the set point of the thermostat may be adjusted according to the occupancy of the occupancy space. For example, when heating a home or office, the set point is generally lowered in the evening, when the homeowners are sleeping or the office is unoccupied, in order to conserve energy. The most common method of adjusting the set point of the thermostat is by using a timer which adjusts the set point according to the time and day of the week when the office or home is likely to be unoccupied. These type of thermostats are very effective when the occupancy of the occupancy space is predictable. However, these thermostats are less effective when the occupancy continually changes. Furthermore, these timed thermostats can not be easily reprogrammed nor is it practical to continually reprogram the thermostat for varying occupancy rate, especially for offices where the employees may periodically work on weekends or holidays.
To overcome the drawbacks of the timed thermostats, a thermostat may sense a condition that is indicative of the occupancy of the occupancy space, such as the motion, sound, light, water flow in the plumbing and voltage loads in the house or office, as shown in U.S. Pat. No. 5,088,645 to Bell. LIGHTSTAT thermostats are made by the assignee herein so that the set point of the thermostat can be adjusted in accordance with the intensity level of the ambient light present in the occupancy space. Such a light sensing thermostat functions under the premise that if the ambient light is above a predetermined intensity level, such as when the lights in the room are illuminated, then the room is occupied, and when the room is below the predetermined intensity level, the room is unoccupied. Such thermostats cannot distinguish between natural light and artificial light. The referenced thermostats must be located away from windows or direct sunlight to provide an unambiguous indication of the occupancy of the room. Since these thermostats are particularly difficult to adjust for rooms that are well lit by natural light, there is a need for a thermostat that distinguishes between daylight and artificial light.
Accordingly, it is an object of the present invention to provide a light sensitive thermostat that adjusts the temperature set point in a fashion better related to the occupancy of the occupancy space by detecting as having first the presence of artificial light, and then providing an output related only to the presence of artificial light.
It is another object to provide a thermostat that provides a more accurate indication of the occupancy of a room, and adjusts the thermostat's set point accordingly.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.